You're not wrong, but you're not thinking like a shithead DA.

Rat poison was taken from the floor and added to the food of a police officer. We don't know who exactly did it, but everyone in the kitchen could reasonably have known it was happening and did nothing to stop it, and therefore is guilty of felony murder.

Again, the DA doesn't need to be right, they just need to convince a grand jury that there's a chance they're right, then they force the accused in to a plea deal. Because these are kitchen guys, they don't make much money, so they'll be using a public defender who will likely tell them they're going to prison, and the best thing they can do is plea out.